The invention pertains to the field of ashing of solid fuels or combusted or partly combusted fuels, or any organic and inorganic matrix which contains minerals, e.g. oil and human blood, at low to very low temperature. All solid fuels, whether they consist of renewal biomass, fossil fuels such as coal and peat or fuels in gas phase or liquid phase, am based on carbon, hydrogen, oxygen and nitrogen. In addition these elements, solid fuels also contain minerals such as sulphur, silica and metals, often connected to oxygen. Minerals with the uncombusted matter constitute the ash, which consists of various oxides, sulphates and silicates as well as other chemical compounds. Table Ia shows an example of the composition of an ash from coal obtained by chemical analysis. Table Ib show an example of the most common minerals in coal.
TABLE Ia ______________________________________ SiO.sub.2 25-50% Al.sub.2 O.sub.3 10-40% Fe.sub.2 O.sub.3 5-30% CaO 1-15% MgO 0.5-5% TiO.sub.2 0.5-3% Na.sub.2 O + K.sub.2 O 1-4% SO.sub.3 0.1-10% ______________________________________
TABLE Ib ______________________________________ Silicates (Quartz SiO.sub.2, Feldspar (K, Na)(AlSi.sub.3 O.sub.8), CaAl.sub.2 Si.sub.2 O.sub.8) Clayminerals (Illite K.sub.1-1.5 Al.sub.4 (Si.sub.7-6.5 Al.sub.1-1.5 O.sub.20)(OH).sub.4, Kaolinite Al.sub.4 (Si.sub.4 O.sub.10)(OH).sub.8, Montmorillonite (1/2 Ca, Na).sub.0.7 (Al, Mg, Fe).sub.4 ((Si, Al).sub.8 O.sub.20)(OH).sub.4.n H.sub.2 O) Sulphates (Anhydrite CaSO.sub.4, Barite BaSO.sub.4) Sulphides (Pyrite FeS.sub.2, Marcasite FeS.sub.2, Sphalerite ZnS) Elemental Sulphur S.sub.8 Carbonates (Calcite CaCO.sub.3, Siderite FeCO.sub.3) ______________________________________
On combustion ashes are formed along with the gases, which principally are carbon monooxide and carbon dioxide. Furthermore, nitrogen oxides (NO,N.sub.2 O,NO.sub.2) are formed by oxidation of nitrogen in the fuel or in the air. Sulphur dioxide and sulphur trioxide are also present in the gases. The sulphur oxides are formed by the oxidation of sulphur in coal, which is present in three modes, namely organic sulphur, sulphides and sulphates. The sulphides and sulphates are counted as mineral matter in the fuels.
It is desirable to choose fuel with low sulphur content, whereby the emission of sulphur oxides decreases.
It is seen from the foregoing that it is important that the selection of the fuel is not based simply on its specific heat, but also on a knowledge of the composition of the mineral matter.
The performance of chemical analyses on the ash are well known and the customary procedure is first to ash the sample, which means that the organic matter in the fuel oxidizes at high temperature in the presence of oxygen (air). The obtained ash can be analyzed in a relevant manner.
The ash can also be mounted in the mounting material and a cross-section of the ash can be analyzed using an electron microscope. In this case it is possible to analyze the ash by x-ray methods.
In order to perform X-ray analysis the ashing of the samples should have proceeded as long as possible before the mounting of the ashes is carried out.
The compounds in the sample usually oxidize at high temperature and high oxygen content. For example, iron which usually exists as Fe.sup.2+ compounds, is oxidized to Fe.sub.2 O.sub.3. Iron sulphides and copper sulphides oxidize to sulphates. Calcium oxide transforms to calcium sulphate by reaction with SO.sub.2, which originates from the just mentioned sulphides. Consequently, the original composition of the mineral matter in the fuel is not reflected by the results of the chemical analysis.
To be able to study the ash more thoroughly, the phase composition of the minerals is essential. Table Ib show an example of the minerals which usually occur in solid fuels.
It is important to find methods in which ashing of the fuel sample can be performed without phase modifications or losses of the minerals at as low a temperature as possible. One of these methods is Low Temperature. Ashing (LTA) which will be described below.
Since 1965 it has been known (Gluskoter, Fuel, 44, 285-291) that by applying an radio frequency field to a gas atmosphere containing 100% oxygen a plasma is obtained, that is a gas containing electrically conductive ions, and this plasma has the ability to oxidize carbon bound to other organic and inorganic material.